JSONPath support for "step" expression on array slice

[JohannesLipp]

I need to select every n-th element from a JSON array, but it seems that the JSONPath array slice expression "step" is not supported. An example would be [0:10:2] to select every other element in the range 0-9.

Input JSON: { "array": [0,1,2,3,4,5,6,7,8,9] }

RML mapping:

@prefix :     <http://example.org/> .
@prefix ql:   <http://semweb.mmlab.be/ns/ql#>.
@prefix rml:  <http://semweb.mmlab.be/ns/rml#>.
@prefix rr:   <http://www.w3.org/ns/r2rml#>.
@prefix xsd:  <http://www.w3.org/2001/XMLSchema#> .

:Map
    a rr:TriplesMap ;
    rml:logicalSource [
        rml:source               "/src/main/resources/ex.json" ;
        rml:referenceFormulation ql:JSONPath ;
        rml:iterator             "$" ] ;

    rr:subjectMap [ rr:termType rr:BlankNode ] ;

    rr:predicateObjectMap [rr:predicate :hasEntry; rr:objectMap [rml:reference "array[0:10:2]"; rr:datatype xsd:integer]] .

Actual output:

@prefix : <http://example.org/> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .

_:0 :hasEntry 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 .

Expected output:

@prefix : <http://example.org/> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .

_:0 :hasEntry 0, 2, 4, 6, 8 .

Is there anything we can do about this? Thank you very much in advance!

[DylanVanAssche]

Hi @JohannesLipp !

Thank you for your interest in RML and our tools! The JSONPath specification you refer to is still a draft at IETF which was recently being established from https://goessner.net/articles/JsonPath/. The JSONPath library inside the RMLMapper does not support the new specification yet and therefore not the array slice expression.

You may be able to workaround this issue using FnO functions. The RMLMapper supports a default set of functions, but you can also add your own functions. You can find the default set of functions at: https://rml.io/docs/rmlmapper/default-functions/

If you still have any questions, feel free to reach out!

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[JohannesLipp]

Hi @DylanVanAssche thank you very much for your quick reply! We investigated FnO functions in the RML Mapper and successfully implemented these in other use-cases, but: We evaluated the required effort for the JSON indexing as too high, so we actually changed the format of our input files to enable easy processing via default RML JSON-path.

Concretely, we split the list in the input file into two lists containing every other element. Thank you anyway! 🦀